1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor integrated circuits having thin film transistors formed on an insulating surface. In the context of the present invention, the term "insulating surface" means an insulating substrate, an insulating film formed thereon, or an insulating film formed on a material such as a semiconductor and metal. More particularly, the present invention relates to semiconductor integrated circuits which employ a metal material mainly composed of aluminum as the material for gate electrodes and gate lines, such as active matrix circuits used for liquid crystal displays.
2. Description of the Related Art
Thin film transistors (TFTs) have been manufactured using a self-alignment process with the aid of single crystal semiconductor integrated circuit techniques. According to this process, a gate electrode is formed on a semiconductor film through a gate insulation film and impurities are introduced into the semiconductor film using the gate electrode as a mask. Impurities can be introduced using methods such as thermal diffusion, ion implantation, plasma doping, and laser doping.
Conventionally, the gate electrodes of TFTs have employed silicon having conductivity enhanced by doping with the aid of single crystal semiconductor circuit techniques. This material has high heat resistance characteristics and hence has been an idealistic material in a case wherein a high temperature process is performed. However, it has been recently found that the use of a silicon gate is not appropriate.
The first reason is that silicon has low conductivity. This problem has not been significant in devices having a relatively small surface area. However, it has become significant because the increasing size of liquid crystal displays has resulted in increases in the size of active matrix circuits and design rules (the widths of gate lines) left unchanged.
The second reason is that as the size of devices has been increased, it has become necessary to switch the material for substrates from expensive materials having high heat resistance characteristics such as quartz and silicon wafers to less expensive materials having lower heat resistance characteristics such as the glass available from Corning Corp. under product No. 7059 and the borosilicate glass available from NH Technoglass Corp. under product NA-35, NA-45 etc. Such materials have not been appropriate as materials for substrates because the formation of silicon gates involves a heating process at 650.degree. C. or higher.
In view of such a problem, it has been necessary that the silicon gates must be replaced by aluminum gates. In this case, although pure aluminum may be used, a material such as silicon, copper and scandium (Sc) is added in a small amount because pure aluminum exhibits extremely low heat resistance characteristics. Even with such an additive, aluminum still has the problem with heat resistance characteristics. Therefore, for aluminum gates, it has not been possible to use a thermal annealing process to activate impurities after a doping process such as ion implantation utilizing accelerated ions, and optical annealing utilizing, e.g. laser irradiation has been employed for such a purpose. Even in the latter case, severe limitations have been placed on the intensity of the light to irradiate the aluminum gates in order to prevent damage to the gates by the laser irradiation.
Aluminum itself reflects light in a wide range of wavelengths including ultraviolet rays and infrared rays if it has a mirror surface. However, the use of aluminum, has not been appropriate, for example, where flash-lamp annealing is employed. The reason is this process involves irradiation for a long time which results in a rise in the temperature of a silicon film caused by the light absorbed by the silicon film or the like and the temperature rise transferred to the aluminum as a result of thermal conduction causes melting and deformation of the aluminum. The same problem has been encountered in laser annealing and in a method wherein continuously oscillated laser beams are irradiated. When extremely short oscillation pulse laser is irradiated, light absorbed by a silicon film operates on only annealing for the silicon film, so that the aluminum can be used without increasing the temperature of the aluminum.
FIGS. 4A to 4E show steps for manufacturing thin film transistors having an aluminum gate based on the above-described considerations. First, a backing insulation film 402 is formed on a substrate 401 and island-like crystalline semiconductor regions 403 and 404 are formed thereon. An insulation film 405 which serves as a gate insulation film is formed to cover those regions. (FIG. 4A.)
Then, gate electrodes/gate lines 406 and 407 are formed by using a material mainly composed of aluminum. (FIG. 4B).
Next, impurities (e.g., phosphorous (P) or boron (B)) are implanted on a self-alignment using the gate electrodes/gate lines 406 and 407 as masks according to the ion implantation method, ion doping method, or the like to form impurity regions 408 and 409. In this case, phosphorous is implanted in the impurity region 408 and boron is implanted in the impurity region 409. As a result, the former becomes an N-type region and the latter becomes a P-type region. (FIG. 4C.)
Thereafter, a pulse laser beam is directed from the upper side to active the regions where impurities have been introduced. (FIG. 4D.)
Finally, an interlayer insulator 411 is deposited; a contact hole is formed in each of the impurity regions; electrodes/lines 412 through 416 connected to the contact holes are formed to complete thin film transistors. (FIG. 4E.)
According to the above-described method, however, the boundaries between the impurity regions and regions wherein channels are to be formed (semiconductor regions directly under gate electrodes which are sandwiched by the impurity regions, e.g., the region indicated by 410 in FIG. 4D. are electrically unstable because they have not been subjected to a sufficient treatment during processing. It has been found that those regions create problems such as an increase in a leak current, thereby reducing reliability after use for a long period.
Specifically, as apparent from the processing steps illustrated, neither introduction of impurities nor laser irradiation takes place once a gate electrode is formed. Therefore, substantially no change occurs in the crystallinity of the region where a channel is to be formed.
On the other hand, impurity regions adjacent to a region wherein a channel to be formed initially have the same crystallinity as that of the region wherein a channel is formed. However, the crystallinity is decreased by the introduction of impurities. Although the impurity regions are repaired by a laser irradiation process performed later, it is difficult to obtain the initial crystallinity. Especially, the areas of the impurity regions which are adjacent to the active region can not be sufficiently activated because such areas are not likely to be irradiated with laser light. Specifically, since the crystallinity is discontinuous between the impurity regions and the active region, a trap level or the like produces easily. Especially, when impurities are introduced using a method wherein accelerated ions are applied, impurity ions are dispersed into the area under the gate electrode portion and destroy the crystallinity in that area. It has not been possible to activate such an area under a gaze electrode portion using a laser beam or the like because the gate electrode portion blocks the beam.
This equally applies to the gate insulation film. Specifically, while the gate insulation film above the region wherein a channel is to be formed remains in the initial state, the gate insulation film above the impurity regions undergoes great changes during steps such as introduction of impurities and laser irradiation. As a result, many traps occur at the boundaries between those regions.
One possible solution to this problem is to perform activation by irradiating the substrate on the rear side thereof using a laser or the like. According to this method, since the gate lines are not blocked from the light, the boundaries between the active regions and impurity regions are sufficiently activated. In this case, however, the material of the substrate must transmit light. Since most glass substrates can not easily transmit ultraviolet rays having wavelengths of 300 nm or less, for example, a KrF excimer laser (having a wavelength of 248 nm) that excels in mass productivity can not be used.
Further, during the laser irradiation step as described above, aluminum is heated to a high temperature, although only instantaneously. This has resulted in abnormal growth of aluminum crystals (hillock). Especially, abnormal growth in the vertical direction can cause a short circuit between the aluminum crystals and wiring above them.
When ion doping is carried out to dope impurities, another problem arises. Ion doping is a method wherein a gas including impurities for doping (e.g., phosphine (PH.sub.3) if phosphorous is to be doped and diborane (B.sub.2 H.sub.6) if boron is to be doped) is subjected to electrical discharge and resulting ions are taken out and emitted using a high voltage.
This method is simpler compared to ion implantation and is suitable for processing a large surface area. According to this method, however, various ions are emitted because mass separation is not performed. Especially, a very large amount of hydrogen ions are emitted both in atomic and molecular states. If such hydrogen ions exist in the gate insulation film in the vicinity of a gate electrode (the gate insulation film above the region 410), fluctuations in characteristics can be caused when a voltage is applied. Especially, the method shown in FIGS. 4A to 4E has had a problem in that hydrogen implanted in a gate electrode can not be sufficiently removed.